Structural determination of oleanane-28,13β-olide and taraxerane-28,14β-olide fluorolactonization products from the reaction of oleanolic acid with SelectfluorTM

X-ray analysis and structure determination of fluorolactonization products from the reaction of oleanolic acid with SelectfluorTM are reported.


Chemical context
Oleanolic acid (OA) and ursolic acid (UA) are pentacyclic triterpenoids that are found widely in food and plants of the Oleaceae family, such as the olive plant.Similar to many other natural products, these triterpenoids have been found to exhibit a range of pharmacological activities (Sa ´nchez- Quesada et al., 2013), such as antioxidant, anti-inflammatory (Adjei et al., 2021), anti-diabetic (Qian et al., 2010;Tang et al., 2014), and anti-cancer properties (Borella et al., 2019;Baer-Dubowska et al., 2021).Previous reports have demonstrated that semi-synthetic derivatives of UA and OA-fluorolactones have improved biological activities compared to the parent molecules, demonstrating both anti-apoptotic (Leal et al., 2012) and anti-diabetic properties (Zhong et al., 2019).Leal and co-workers report that the reaction of UA with Selectfluor TM at 353 K, in a mixture of nitromethane and dioxane, results in insertion of fluorine at C-12 with formation of the 28,13�-�-lactone.The authors report that fluorolactonization of UA gives a mixture of � and �-isomers (C-F), with the �-isomer formed as the major product, as characterized by 1 H NMR spectroscopy (Leal et al., 2012).Zhong and co-workers (Zhong et al., 2019) also report fluorolactonization of OA, under these same conditions (Leal et al., 2012), to give fluorination at C-12 and formation of the 28,13�-�-lactone.However, the authors (Zhong et al., 2019) do not comment on the stereochemistry at the C-F bond (C-12).
Given the previous reports indicating enhanced pharmacological properties in UA and OA-fluorolactone derivatives (Leal et al., 2012;Zhong et al., 2019), our research pursuits include the additional functionalization of OA-fluorolactones.This exploration aims to unveil alternative biological activities within this class of compounds (Eadsforth, 2022).We there-fore adopted the same conditions as previously described for UA (Leal et al., 2012) and OA (Zhong et al., 2019) for the fluorolactonization of OA (see scheme).Interestingly, we found that under these conditions a mixture of fluorolactone products was formed, including the 28,14�-�-lactone, which has not previously been characterized (Fig. 1).Herein, we report on the synthesis and crystal structures of the products of the fluorolactonization reaction of OA with Selectfluor TM , an electrophilic fluorinating reagent, which leads to rearrangement to a taraxerane core as the major product.Taraxeranes are another class of biologically active pentacyclic triterpenoid steroids that have been isolated from plants.Taraxeranes are structural isomers of oleanane triterpenoids that are derived bio-synthetically from the rearrangement of the oleanane skeleton following the C-27 methyl shift from C-14 to C-13 (Kuroda et al., 2006;Hu et al., 2012).
Since acid catalysis is unlikely under the electrophilic fluorinating conditions employed with Selectfluor TM and given that we failed to isolate any intermediates containing a C-14 C-15 double bond, we propose that formation of (2) follows a different mechanism to that proposed for the bromolactonization of OA (Martinez et al., 2015).We propose that following electrophilic addition of fluorine to the C12 C13 double bond, the tertiary carbocation formed at C-13 can either be stabilized by the intramolecular nucleophilic attack of the C-28 carboxyl to form (1) or by the Wagner-Meerwein 1,2-shift of C-27 to C-13.This rearrangement would result in a tertiary carbocation at C-14, which can then be stabilized by the nucleophilic attack of the C-28 carboxyl group to form the 28,14�-�-lactone (2) (Fig. 1).
X-ray analysis confirmed that the unusual major product, compound (2), contains a �-lactone ring; in (2), the C28 O2 carbonyl is adjacent to C-17 and the bridging oxygen atom O-1 adjacent to C-14 (Fig. 3).The ether oxygen displays a �-configuration, while the methyl group has an �-configuration.The structure shows that the methyl group C-27 has undergone a 1,2-shift from C-14 to C-13, retaining its original axial orientation from oleanolic acid.The fluorine atom at C-12, belonging to ring C, is oriented equatorially and assumes an �-configuration.Rings A, B and E of the triterpenoid skeleton adopt chair conformations [Cremer & Pople, 1975; puckering parameters: The molecular structure of (1), showing the atomic labelling scheme.Non-H atoms are drawn as 50% probability displacement ellipsoids and H atoms are drawn as spheres of an arbitrary size.Oxygen atoms are coloured in red, fluorine atom coloured in green.

Figure 3
The molecular structure of (2), showing the atomic labelling scheme.Non-H atoms are drawn as 50% probability displacement ellipsoids and H atoms are drawn as spheres of an arbitrary size.Methanol solvent molecule hydrogen bonding to O-3.Oxygen atoms are coloured in red, fluorine atom coloured in green.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. Hydrogen atoms were placed in calculated positions and refined using idealized geometries, except H9 in compound (2), which was allowed to refine freely.Hydrogen isotropic atomic displacement parameters were constrained to ride with the parent atom with an appropriate multiplier for the hybridization.Atomic displacement parameters for C1B-C7B in compound (1) were restrained with a strong isotropic atomic displacement parameter restraint in order to refine chemically sensible atomic displacement parameters.

Special details
Geometry.All esds (except the esd in the dihedral angle between two l.s.planes) are estimated using the full covariance matrix.The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry.An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s.planes.Refinement.Data were streaked.this was the best crystal that could be found.Streaking has resulted in prolate atoms as not all intensity for all resflections could be accurately captured.ISOR restraints applied to refine more realistic thermal parameters for these atoms.Data collection: X-ray diffraction data for compounds (1) and (2) were collected using a dual wavelength Rigaku FR-X rotating anode diffractometer using CuKα (λ = 1.54184Å) radiation, equipped with an AFC-11 4-circle kappa goniometer, VariMAX TM microfocus optics, a Hypix-6000HE detector and an Oxford Cryosystems 800 plus nitrogen flow gas system, at temperatures of 240K and 100K, respectively.Data were collected and reduced using CrysAlisPro v40.Absorption correction was performed using empirical methods (SCALE3 ABSPACK) based upon symmetryequivalent reflections combined with measurements at different azimuthal angles.Crystal structure determination and refinements: The crystal structure was solved and refined against all F 2 values using the SHELX and Olex2 suite of programmes.All non-hydrogen atoms were refined anisotropically.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å
) contain five fused six-membered rings.The X-ray structure of compound (1) revealed that it contains a �-lactone ring with the C28 O2 carbonyl adjacent to C-17 and the bridging oxygen atom O-1 adjacent to C-13.The ether oxygen atom O-1 at C-13 and the methyl group at C-14 are axial with respect to rings C and D. The fluorine atom at C-12, belonging to ring C, is oriented equatorially and assumes an �-configuration.Rings A-E of the triterpenoid skeleton adopt chair conformations, and rings D and E are cis-fused as in oleanolic acid (Fig. 2).The values of the dihedral angles in (1) confirm the trans configuration of rings A/B, B/C and C/D [À 177.9, 172.3, À 170.8 (6) � ] and the cis configuration of rings D/E [65.1 � ].Each six-membered ring adopts a chair conformation with a different degree of distortion, as shown by the Cremer & Pople (1975) parameters: [ring A: Q = 0.552 A ˚, � = 3.8 � and ' = 312 � ; B: Q = 0.547 A ˚, � = 166.2� and ' = 4.6 � ; C: Q = 0.550 A ˚, � = 20.4� and ' = 132.3� ; D: Q = 0.607 A ˚, � = 161.2� and ' = 231.3� ; E: Q = 0.570 A ˚, � = 177.5 � and ' = 247.5 � ].

Figure 4
Figure 4Expanded view of crystal packing of structure (1) to show hydrogen bonding (dotted lines) to solvent methanol.Oxygen atoms are coloured in red, fluorine atom coloured in green.The hydrogen atoms have been omitted for clarity.

Figure 5
Figure 5Expanded view of crystal packing of structure (2) to show hydrogen bonding (dotted lines) to solvent methanol.Oxygen atoms are coloured in red, fluorine atom coloured in green.The hydrogen atoms have been omitted for clarity.

Table 3
Experimental details.